clippy_lints/src/needless_pass_by_value.rs

   1 use crate::utils::ptr::get_spans;
   2 use crate::utils::{
   3     get_trait_def_id, implements_trait, in_macro_or_desugar, is_copy, is_self, match_type, multispan_sugg, paths,
   4     snippet, snippet_opt, span_lint_and_then,
   5 };
   6 use if_chain::if_chain;
   7 use matches::matches;
   8 use rustc::hir::intravisit::FnKind;
   9 use rustc::hir::*;
  10 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
  11 use rustc::middle::expr_use_visitor as euv;
  12 use rustc::middle::mem_categorization as mc;
  13 use rustc::traits;
  14 use rustc::ty::{self, RegionKind, TypeFoldable};
  15 use rustc::{declare_lint_pass, declare_tool_lint};
  16 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
  17 use rustc_errors::Applicability;
  18 use rustc_target::spec::abi::Abi;
  19 use std::borrow::Cow;
  20 use syntax::ast::Attribute;
  21 use syntax::errors::DiagnosticBuilder;
  22 use syntax_pos::Span;
  23
  24 declare_clippy_lint! {
  25     /// **What it does:** Checks for functions taking arguments by value, but not
  26     /// consuming them in its
  27     /// body.
  28     ///
  29     /// **Why is this bad?** Taking arguments by reference is more flexible and can
  30     /// sometimes avoid
  31     /// unnecessary allocations.
  32     ///
  33     /// **Known problems:**
  34     /// * This lint suggests taking an argument by reference,
  35     /// however sometimes it is better to let users decide the argument type
  36     /// (by using `Borrow` trait, for example), depending on how the function is used.
  37     ///
  38     /// **Example:**
  39     /// ```rust
  40     /// fn foo(v: Vec<i32>) {
  41     ///     assert_eq!(v.len(), 42);
  42     /// }
  43     /// // should be
  44     /// fn foo(v: &[i32]) {
  45     ///     assert_eq!(v.len(), 42);
  46     /// }
  47     /// ```
  48     pub NEEDLESS_PASS_BY_VALUE,
  49     pedantic,
  50     "functions taking arguments by value, but not consuming them in its body"
  51 }
  52
  53 declare_lint_pass!(NeedlessPassByValue => [NEEDLESS_PASS_BY_VALUE]);
  54
  55 macro_rules! need {
  56     ($e: expr) => {
  57         if let Some(x) = $e {
  58             x
  59         } else {
  60             return;
  61         }
  62     };
  63 }
  64
  65 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessPassByValue {
  66     #[allow(clippy::too_many_lines)]
  67     fn check_fn(
  68         &mut self,
  69         cx: &LateContext<'a, 'tcx>,
  70         kind: FnKind<'tcx>,
  71         decl: &'tcx FnDecl,
  72         body: &'tcx Body,
  73         span: Span,
  74         hir_id: HirId,
  75     ) {
  76         if in_macro_or_desugar(span) {
  77             return;
  78         }
  79
  80         match kind {
  81             FnKind::ItemFn(.., header, _, attrs) => {
  82                 if header.abi != Abi::Rust || requires_exact_signature(attrs) {
  83                     return;
  84                 }
  85             },
  86             FnKind::Method(..) => (),
  87             _ => return,
  88         }
  89
  90         // Exclude non-inherent impls
  91         if let Some(Node::Item(item)) = cx.tcx.hir().find(cx.tcx.hir().get_parent_node(hir_id)) {
  92             if matches!(item.node, ItemKind::Impl(_, _, _, _, Some(_), _, _) |
  93                 ItemKind::Trait(..))
  94             {
  95                 return;
  96             }
  97         }
  98
  99         // Allow `Borrow` or functions to be taken by value
 100         let borrow_trait = need!(get_trait_def_id(cx, &paths::BORROW_TRAIT));
 101         let whitelisted_traits = [
 102             need!(cx.tcx.lang_items().fn_trait()),
 103             need!(cx.tcx.lang_items().fn_once_trait()),
 104             need!(cx.tcx.lang_items().fn_mut_trait()),
 105             need!(get_trait_def_id(cx, &paths::RANGE_ARGUMENT_TRAIT)),
 106         ];
 107
 108         let sized_trait = need!(cx.tcx.lang_items().sized_trait());
 109
 110         let fn_def_id = cx.tcx.hir().local_def_id(hir_id);
 111
 112         let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds.to_vec())
 113             .filter(|p| !p.is_global())
 114             .filter_map(|pred| {
 115                 if let ty::Predicate::Trait(poly_trait_ref) = pred {
 116                     if poly_trait_ref.def_id() == sized_trait || poly_trait_ref.skip_binder().has_escaping_bound_vars()
 117                     {
 118                         return None;
 119                     }
 120                     Some(poly_trait_ref)
 121                 } else {
 122                     None
 123                 }
 124             })
 125             .collect::<Vec<_>>();
 126
 127         // Collect moved variables and spans which will need dereferencings from the
 128         // function body.
 129         let MovedVariablesCtxt {
 130             moved_vars,
 131             spans_need_deref,
 132             ..
 133         } = {
 134             let mut ctx = MovedVariablesCtxt::new(cx);
 135             let region_scope_tree = &cx.tcx.region_scope_tree(fn_def_id);
 136             euv::ExprUseVisitor::new(
 137                 &mut ctx,
 138                 cx.tcx,
 139                 fn_def_id,
 140                 cx.param_env,
 141                 region_scope_tree,
 142                 cx.tables,
 143                 None,
 144             )
 145             .consume_body(body);
 146             ctx
 147         };
 148
 149         let fn_sig = cx.tcx.fn_sig(fn_def_id);
 150         let fn_sig = cx.tcx.erase_late_bound_regions(&fn_sig);
 151
 152         for (idx, ((input, &ty), arg)) in decl.inputs.iter().zip(fn_sig.inputs()).zip(&body.arguments).enumerate() {
 153             // All spans generated from a proc-macro invocation are the same...
 154             if span == input.span {
 155                 return;
 156             }
 157
 158             // Ignore `self`s.
 159             if idx == 0 {
 160                 if let PatKind::Binding(.., ident, _) = arg.pat.node {
 161                     if ident.as_str() == "self" {
 162                         continue;
 163                     }
 164                 }
 165             }
 166
 167             //
 168             // * Exclude a type that is specifically bounded by `Borrow`.
 169             // * Exclude a type whose reference also fulfills its bound. (e.g., `std::convert::AsRef`,
 170             //   `serde::Serialize`)
 171             let (implements_borrow_trait, all_borrowable_trait) = {
 172                 let preds = preds
 173                     .iter()
 174                     .filter(|t| t.skip_binder().self_ty() == ty)
 175                     .collect::<Vec<_>>();
 176
 177                 (
 178                     preds.iter().any(|t| t.def_id() == borrow_trait),
 179                     !preds.is_empty()
 180                         && preds.iter().all(|t| {
 181                             let ty_params = &t
 182                                 .skip_binder()
 183                                 .trait_ref
 184                                 .substs
 185                                 .iter()
 186                                 .skip(1)
 187                                 .cloned()
 188                                 .collect::<Vec<_>>();
 189                             implements_trait(cx, cx.tcx.mk_imm_ref(&RegionKind::ReEmpty, ty), t.def_id(), ty_params)
 190                         }),
 191                 )
 192             };
 193
 194             if_chain! {
 195                 if !is_self(arg);
 196                 if !ty.is_mutable_pointer();
 197                 if !is_copy(cx, ty);
 198                 if !whitelisted_traits.iter().any(|&t| implements_trait(cx, ty, t, &[]));
 199                 if !implements_borrow_trait;
 200                 if !all_borrowable_trait;
 201
 202                 if let PatKind::Binding(mode, canonical_id, ..) = arg.pat.node;
 203                 if !moved_vars.contains(&canonical_id);
 204                 then {
 205                     if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut {
 206                         continue;
 207                     }
 208
 209                     // Dereference suggestion
 210                     let sugg = |db: &mut DiagnosticBuilder<'_>| {
 211                         if let ty::Adt(def, ..) = ty.sty {
 212                             if let Some(span) = cx.tcx.hir().span_if_local(def.did) {
 213                                 if cx.param_env.can_type_implement_copy(cx.tcx, ty).is_ok() {
 214                                     db.span_help(span, "consider marking this type as Copy");
 215                                 }
 216                             }
 217                         }
 218
 219                         let deref_span = spans_need_deref.get(&canonical_id);
 220                         if_chain! {
 221                             if match_type(cx, ty, &paths::VEC);
 222                             if let Some(clone_spans) =
 223                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
 224                             if let TyKind::Path(QPath::Resolved(_, ref path)) = input.node;
 225                             if let Some(elem_ty) = path.segments.iter()
 226                                 .find(|seg| seg.ident.name == sym!(Vec))
 227                                 .and_then(|ps| ps.args.as_ref())
 228                                 .map(|params| params.args.iter().find_map(|arg| match arg {
 229                                     GenericArg::Type(ty) => Some(ty),
 230                                     _ => None,
 231                                 }).unwrap());
 232                             then {
 233                                 let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
 234                                 db.span_suggestion(
 235                                     input.span,
 236                                     "consider changing the type to",
 237                                     slice_ty,
 238                                     Applicability::Unspecified,
 239                                 );
 240
 241                                 for (span, suggestion) in clone_spans {
 242                                     db.span_suggestion(
 243                                         span,
 244                                         &snippet_opt(cx, span)
 245                                             .map_or(
 246                                                 "change the call to".into(),
 247                                                 |x| Cow::from(format!("change `{}` to", x)),
 248                                             ),
 249                                         suggestion.into(),
 250                                         Applicability::Unspecified,
 251                                     );
 252                                 }
 253
 254                                 // cannot be destructured, no need for `*` suggestion
 255                                 assert!(deref_span.is_none());
 256                                 return;
 257                             }
 258                         }
 259
 260                         if match_type(cx, ty, &paths::STRING) {
 261                             if let Some(clone_spans) =
 262                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
 263                                 db.span_suggestion(
 264                                     input.span,
 265                                     "consider changing the type to",
 266                                     "&str".to_string(),
 267                                     Applicability::Unspecified,
 268                                 );
 269
 270                                 for (span, suggestion) in clone_spans {
 271                                     db.span_suggestion(
 272                                         span,
 273                                         &snippet_opt(cx, span)
 274                                             .map_or(
 275                                                 "change the call to".into(),
 276                                                 |x| Cow::from(format!("change `{}` to", x))
 277                                             ),
 278                                         suggestion.into(),
 279                                         Applicability::Unspecified,
 280                                     );
 281                                 }
 282
 283                                 assert!(deref_span.is_none());
 284                                 return;
 285                             }
 286                         }
 287
 288                         let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
 289
 290                         // Suggests adding `*` to dereference the added reference.
 291                         if let Some(deref_span) = deref_span {
 292                             spans.extend(
 293                                 deref_span
 294                                     .iter()
 295                                     .cloned()
 296                                     .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
 297                             );
 298                             spans.sort_by_key(|&(span, _)| span);
 299                         }
 300                         multispan_sugg(db, "consider taking a reference instead".to_string(), spans);
 301                     };
 302
 303                     span_lint_and_then(
 304                         cx,
 305                         NEEDLESS_PASS_BY_VALUE,
 306                         input.span,
 307                         "this argument is passed by value, but not consumed in the function body",
 308                         sugg,
 309                     );
 310                 }
 311             }
 312         }
 313     }
 314 }
 315
 316 /// Functions marked with these attributes must have the exact signature.
 317 fn requires_exact_signature(attrs: &[Attribute]) -> bool {
 318     attrs.iter().any(|attr| {
 319         [sym!(proc_macro), sym!(proc_macro_attribute), sym!(proc_macro_derive)]
 320             .iter()
 321             .any(|&allow| attr.check_name(allow))
 322     })
 323 }
 324
 325 struct MovedVariablesCtxt<'a, 'tcx> {
 326     cx: &'a LateContext<'a, 'tcx>,
 327     moved_vars: FxHashSet<HirId>,
 328     /// Spans which need to be prefixed with `*` for dereferencing the
 329     /// suggested additional reference.
 330     spans_need_deref: FxHashMap<HirId, FxHashSet<Span>>,
 331 }
 332
 333 impl<'a, 'tcx> MovedVariablesCtxt<'a, 'tcx> {
 334     fn new(cx: &'a LateContext<'a, 'tcx>) -> Self {
 335         Self {
 336             cx,
 337             moved_vars: FxHashSet::default(),
 338             spans_need_deref: FxHashMap::default(),
 339         }
 340     }
 341
 342     fn move_common(&mut self, _consume_id: HirId, _span: Span, cmt: &mc::cmt_<'tcx>) {
 343         let cmt = unwrap_downcast_or_interior(cmt);
 344
 345         if let mc::Categorization::Local(vid) = cmt.cat {
 346             self.moved_vars.insert(vid);
 347         }
 348     }
 349
 350     fn non_moving_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>) {
 351         let cmt = unwrap_downcast_or_interior(cmt);
 352
 353         if let mc::Categorization::Local(vid) = cmt.cat {
 354             let mut id = matched_pat.hir_id;
 355             loop {
 356                 let parent = self.cx.tcx.hir().get_parent_node(id);
 357                 if id == parent {
 358                     // no parent
 359                     return;
 360                 }
 361                 id = parent;
 362
 363                 if let Some(node) = self.cx.tcx.hir().find(id) {
 364                     match node {
 365                         Node::Expr(e) => {
 366                             // `match` and `if let`
 367                             if let ExprKind::Match(ref c, ..) = e.node {
 368                                 self.spans_need_deref
 369                                     .entry(vid)
 370                                     .or_insert_with(FxHashSet::default)
 371                                     .insert(c.span);
 372                             }
 373                         },
 374
 375                         Node::Stmt(s) => {
 376                             // `let <pat> = x;`
 377                             if_chain! {
 378                                 if let StmtKind::Local(ref local) = s.node;
 379                                 then {
 380                                     self.spans_need_deref
 381                                         .entry(vid)
 382                                         .or_insert_with(FxHashSet::default)
 383                                         .insert(local.init
 384                                             .as_ref()
 385                                             .map(|e| e.span)
 386                                             .expect("`let` stmt without init aren't caught by match_pat"));
 387                                 }
 388                             }
 389                         },
 390
 391                         _ => {},
 392                     }
 393                 }
 394             }
 395         }
 396     }
 397 }
 398
 399 impl<'a, 'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt<'a, 'tcx> {
 400     fn consume(&mut self, consume_id: HirId, consume_span: Span, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
 401         if let euv::ConsumeMode::Move(_) = mode {
 402             self.move_common(consume_id, consume_span, cmt);
 403         }
 404     }
 405
 406     fn matched_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::MatchMode) {
 407         if let euv::MatchMode::MovingMatch = mode {
 408             self.move_common(matched_pat.hir_id, matched_pat.span, cmt);
 409         } else {
 410             self.non_moving_pat(matched_pat, cmt);
 411         }
 412     }
 413
 414     fn consume_pat(&mut self, consume_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
 415         if let euv::ConsumeMode::Move(_) = mode {
 416             self.move_common(consume_pat.hir_id, consume_pat.span, cmt);
 417         }
 418     }
 419
 420     fn borrow(
 421         &mut self,
 422         _: HirId,
 423         _: Span,
 424         _: &mc::cmt_<'tcx>,
 425         _: ty::Region<'_>,
 426         _: ty::BorrowKind,
 427         _: euv::LoanCause,
 428     ) {
 429     }
 430
 431     fn mutate(&mut self, _: HirId, _: Span, _: &mc::cmt_<'tcx>, _: euv::MutateMode) {}
 432
 433     fn decl_without_init(&mut self, _: HirId, _: Span) {}
 434 }
 435
 436 fn unwrap_downcast_or_interior<'a, 'tcx>(mut cmt: &'a mc::cmt_<'tcx>) -> mc::cmt_<'tcx> {
 437     loop {
 438         match cmt.cat {
 439             mc::Categorization::Downcast(ref c, _) | mc::Categorization::Interior(ref c, _) => {
 440                 cmt = c;
 441             },
 442             _ => return (*cmt).clone(),
 443         }
 444     }
 445 }